The present invention relates to oil wells and particularly wells that are produced by beam pumping units. Wells that are produced by beam pumping units are commonly referred to as rod pumped wells to distinguish them from wells employing gas-lift or hydraulic pumps to produce the crude oil. In rod pumped wells, it is desirable to shut down the pumping unit when the well has pumped-off and allow the well to recover before restarting the pump. A well is said to be pumped-off when the downhole pump capacity significantly exceeds the volume of fluid available in the wellbore. In this case, the pump barrel does not completely fill with liquid on the up stroke and on the subsequent down stroke, the pump load is not released until the plunger encounters fluid in the barrel. At this point the load is suddenly released which causes pounding that can damage subsurface and surface equipment. Thus, it is desirable to sense when the well pumps-off so that it can be shut in until the additional fluid accumulates. This also results in a power saving and more efficient production.
The simplest way to accomplish this is to periodically test the well and then set the pumping unit on an appropriate duty cycle of on and off periods. If all well conditions remained unchanged, this method would be satisfactory, but well conditions change. Other methods have been suggested in the past such as sensing the electrical power input to electric motors that are used to drive some of the pumping units. When the electrical power decreases, the well is assumed to be pumped-off and the pumping unit is stopped. This approach has the major disadvantage of not measuring the actual power input to the downhole pump. Further, this approach cannot be used on pumping units utilizing internal combustion engines as their prime movers.
Recently, as illustrated by U.S. Pat. Nos. 3,817,094; 3,824,851 and 3,838,597 there has been a renewed interest in utilizing other methods of determining when a well is pumped-off. These patents all utilize some means such as a strain gage for measuring the strain in the beam of the pump and then utilizing the time derivative of this signal to determine when the well is pumped-off.
In the present inventor's previous U.S. Pat. No. 3,343,409 there is disclosed and described a method for determining the performance of a rod pumped well. The method disclosed utilizes a load transducer for measuring the load on the sucker rod in combination with a displacement transducer for measuring the displacement of the rod. Utilizing these two measurements, it is possible to calculate by means of conventional wave theory the actual performance of the downhole pump. The present invention utilizes the same type of measurements to provide a unique method for determining when the well is pumped-off.